The storage of electrical energy by electrochemical energy storage systems such as electrochemical capacitors (supercapacitors) or electrochemical primary or secondary batteries has been available for many years. In this context, the above-mentioned energy storage systems differ in the principle forming the basis of the energy storage.
Generally, supercapacitors include a negative and a positive electrode, which are separated from each other by a separator. In addition, an electrolyte that is ionically conductive is situated between the electrodes. The storage of electrical energy is based on the formation of an electrochemical double layer on the surfaces of the electrodes of the supercapacitor in response to applying a voltage to them. This double layer is formed by solvated charge carriers from the electrolyte, the charge carriers positioning themselves on the surfaces of the oppositely electrically charged electrodes. A redox reaction is not involved in this type of energy storage. Therefore, supercapacitors may theoretically be recharged as much as desired and consequently have a very long service life. The power density of the supercapacitors is also high, whereas the energy density is rather low in comparison with, for example, lithium ion batteries.
However, storage of energy in primary and secondary batteries takes place via a redox reaction. In this context, these batteries also include, generally, a negative and a positive electrode that are separated from each other by a separator. A conductive electrolyte is situated between the electrodes, as well. In lithium ion batteries, one of the most widely used types of secondary batteries, energy is stored by intercalation of lithium ions into the active electrode materials. During operation of the battery cell, that is, during discharge, electrons flow in an external electrical circuit, from the negative electrode to the positive electrode. In a discharge operation, lithium ions migrate, within the battery cell, from the negative electrode to the positive electrode. In this context, the lithium ions are reversibly removed from the active material of the negative electrode, which is also referred to as delithiation. In a charging process of the battery cell, lithium ions migrate from the positive electrode to the negative electrode. In this context, the lithium ions intercalate again into the active material of the negative electrode in a reversible manner, which is also referred to as lithiation.
Lithium ion batteries are characterized in that they have a high energy density, that is, they are able to store a large amount of energy per unit mass or volume. In exchange, however, they only have a limited power density and service life. This is disadvantageous for many applications, which means that in these areas, lithium ion batteries may not be used or only used to a small extent.
Hybrid supercapacitors represent a combination of these technologies and are capable of closing the gap in the potential uses that lithium battery technology and supercapacitor technology have.
Generally, hybrid supercapacitors also have two electrodes, which each include a current diverter and are separated from each other by a separator. The transport of the electrical charges between the electrodes is ensured by electrolytes or electrolytic compounds. Generally, the electrodes include, as an active material, a conventional supercapacitive material (also referred to below as a statically capacitive active material), which is capable of entering into a redox reaction with the charge carriers of the electrolyte and forming an intercalation compound out of them (also referred to below as an electrochemical, redox-active material). Therefore, the energy storage principle of the hybrid supercapacitors is based on the formation of an electrochemical double layer in combination with the formation of a faradic lithium intercalation compound. The energy storage system obtained in this manner has a high energy density, with a simultaneously high power density and long service life.
In light of efforts to utilize energy as completely as possible, there is a need for energy storage systems, which provide as high an energy storage efficiency as possible, in order to be able to store even small quantities of energy with as low a loss as possible.
The use of electrolyte additives for improving the properties of electrolytic compounds for lithium ion batteries is described in the related art, for example, in the Journal of Power Sources 162 (2006), 1379-1394. In Electrochimica Acta, 55 (2010) 3307-3311, Baek describes that 1,3,5-trifluorobenzene may increase the transfer rate of lithium cations at the cathode of lithium ion batteries.
U.S. Pat. No. 8,148,017 describes the use of fluorinated benzene derivatives as an electrolyte additive in double-layer capacitors. The benzene derivatives are selected from hexafluorobenzene, pentafluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene and 1,2,3-trifluorobenzene.
PCT Application No. WO 2015/043923 describes electrolyte compounds, including lithium hexafluorophosphate (LiPF6) and a mixture of a cyclic carbonate and an acyclic carbonate, as well as their use as a solvent or solvent additive for lithium ion batteries and supercapacitors.
An object of the present invention is to provide an electrochemical energy storage system, which has both a high power density and service life and simultaneously stores energy particularly efficiently. This object may be achieved by example embodiment of the present invention, as described below.